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4.4 Recrystallization

4.4.1 General approach

4.4.2 Procedure

  1. Select a suitable solvent:
    1. The most commonly used solvents in order of increasing polarity are petroleum ether, toluene, chloroform, acetone, ethyl acetate, ethanol and water. Chloroform and dichloromethane are rarely useful on their own because they are good solvents for the great majority of organic compounds.
    2. It is preferable to use a solvent with a boiling point in excess of 60ºC, but the b.p. should be at least 10ºC lower than the m.p. of the compound to be crystallized, in order to prevent the solute from “oiling out” of solution.
  2. Dissolving the solute in the minimum volume of hot solvent:
    1. Add a small portion of boiling solvent to the beaker that contains the impure sample and a boiling chip.
    2. Heat the beaker containing the solute and continue adding boiling solvent incrementally until all of the solute has been dissolved.  If additional solvent can be added with no appreciable change in the amount of solute present, the particulate matter is probably insoluble impurities.
  3. Hot Gravity Filtration:
    1. This step is optional if there is no visible particulate matter and the solution is the expected color (most organic compounds are white or light yellow)
    2. If the solution is not the expected color, remove the boiling solution from the heat and allow it to cool to beneath the boiling point of the solvent.  Add a small amount of activated carbon (about the size of a pea) and mix the solution.  If too much activated carbon is used, excessive loss of the desired product will result.  Boil the solution containing the activated carbon for 5 to 10 minutes.  A filter aid will need to be placed in the filter paper to remove the carbon in the following steps.
    3. Flute a piece of filter paper and place it inside of a steamless funnel. A funnel with a stem is prone to premature recrystallization inside the stem because the filtrate can cool as it passes through the stem. At these cooler temperatures, crystals are likely to form.
    4. Heat a beaker that contains some of your recrystallization solvent. Place the funnel and filter paper assembly in the beaker so that the rising vapors from the boiling solvent can heat the funnel and filter paper. Having the set up heated before filtration will prevent crystals from forming on the paper and in the funnel (see figure).

Hot gravity filtration - Keeping the set up hot prevents crystals from forming prematurely.

    1. Keeping the solution very hot so the solute stays dissolved, pour the solution through the funnel and filter paper assembly.  As the filtrate begins to accumulate, heat the receptacle beaker; the resulting vapors will help to prevent any crystallization in the funnel or on the filter paper.
    2. If the funnel was properly heated before filtration, all of the solution will have passed through and no crystals will have formed on the paper or in the funnel. If crystals have formed, pouring a small amount of boiling solvent through the funnel will dissolve these. If the solution is still discolored after using activated carbon and filtering, either the color is from the compound and will not go away or you need to repeat the step with the addition of activated carbon.
    3. The solution should be allowed to cool slowly to room temperature.  Gradual cooling is conducive to the formation of large, well-defined crystals.
  1. Vacuum Filtration:
    1. Agitate the crystals with a fire polished glass-stirring rod before pouring the mother-liquor along with the crystals through the Buchner funnel.  Apply the maximum amount of suction possible using the aspirator.
    2. Some crystals may have been left behind in the beaker; there are two ways to effect a quantitative transfer of all of this material. Either use a portion of the filtrate to rinse the beaker or use a rubber policeman on the end of your stirring rod to scrape the remaining crystals into the Buchner funnel.
    3. When the crystals have been collected and washed, allow the aspirator to run for several minutes so that the crystals have an opportunity to dry.
  2. Drying the Crystals:
    1. When the crystals have been dried as much as possible in the Buchner funnel, use a scopula to remove them to a beaker or crystallizing dish.  This will ensure that the crystals are not contaminated by filter paper fibers as they dry.
    2. After removing all the crystals from the filter paper, remove the filter paper and scrape any remaining crystals from the funnel.
    3. Spreading the crystals out in a beaker or a crystallizing dish will provide for the most efficient drying as the crystals will have a maximum of exposed surface area.
    4. When the crystals are dried, the purity of the sample can be measured by performing a melting point determination.

Click here for - What to do if crystals don't form

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